This application relates to a parallel flow heat exchanger, wherein vapor refrigerant from an upstream location is utilized to provide additional momentum in driving liquid phase refrigerant along a manifold to improve refrigerant distribution among parallel tubes that are in fluid communication with this manifold, and thus enhance the heat exchanger and overall refrigerant system performance.
Refrigerant systems utilize a refrigerant to condition a secondary fluid, such as air, delivered to a climate controlled space. In a basic refrigerant system, the refrigerant is compressed in a compressor, and flows downstream to a condenser, where heat is typically rejected from the refrigerant to ambient environment, during heat transfer interaction with this ambient environment. Then refrigerant flows through an expansion device, where it is expanded to a lower pressure and temperature, and to an evaporator, where during heat transfer interaction with a secondary fluid (e.g., indoor air), the refrigerant is evaporated and typically superheated, while cooling and often dehumidifying this secondary fluid.
In recent years, much interest and design effort has been focused on the efficient operation of the heat exchangers (condenser and evaporator) in the refrigerant systems. One relatively recent advancement in the heat exchanger technology is the development and application of parallel flow, or so-called microchannel or minichannel, heat exchangers (these two terms will be used interchangeably throughout the text), as the condensers and evaporators.
These heat exchangers are provided with a plurality of parallel heat transfer tubes, typically of a non-round shape, among which refrigerant is distributed and flown in a parallel manner. The heat transfer tubes are orientated generally substantially perpendicular to a refrigerant flow direction in the inlet, intermediate and outlet manifolds that are in flow communication with the heat transfer tubes. The primary reasons for the employment of the parallel flow heat exchangers, which usually have aluminum furnace-brazed construction, are related to their superior performance, high degree of compactness, structural rigidity and enhanced resistance to corrosion.
When utilized in condenser applications, these heat exchangers are normally designed for a multi-pass configuration, typically with a plurality of parallel heat transfer tubes within each refrigerant pass, in order to obtain superior performance by balancing and optimizing heat transfer and pressure drop characteristics. In such designs, the refrigerant that enters an inlet manifold (or so-called inlet header) travels through a first multi-tube pass across a width of the condenser to an opposed, typically intermediate, manifold. The refrigerant collected in a first intermediate manifold reverses its direction, is distributed among the heat transfer tubes in the second pass and flows to a second intermediate manifold. This flow pattern can be repeated for a number of times, to achieve optimum condenser performance, until the refrigerant reaches an outlet manifold (or so-called outlet header). Typically, the individual manifolds are of a cylindrical shape (although other shapes are also known in the art) and are represented by different chambers separated by partitions within the same manifold construction assembly.
Heat transfer corrugated and typically louvered fins are placed between the heat transfer tubes for outside heat transfer enhancement and construction rigidity. These fins are typically attached to the heat transfer tubes during a furnace braze operation. Furthermore, each heat transfer tube preferably contains a plurality of relatively small parallel channels for in-tube heat transfer augmentation and structural rigidity.
However, there have been some obstacles to the use of the parallel flow heat exchangers in a refrigerant system. In particular, a problem, known as refrigerant maldistribution, typically occurs in the microchannel heat exchanger manifolds when the two-phase flow enters the manifold. A vapor phase of the two-phase flow has significantly different properties, moves at different velocities and is subjected to different effects of internal and external forces than a liquid phase. This causes the vapor phase to separate from the liquid phase and flow independently. The separation of the vapor phase from the liquid phase has raised challenges, such as refrigerant maldistribution in parallel flow heat exchangers. This phenomenon takes place due to unequal pressure drop inside the channels and in the inlet and outlet manifolds, as well as poor manifold and distribution system design. In the manifolds, the difference in length of refrigerant paths, phase separation and gravity are the primary factors responsible for maldistribution. Inside the heat exchanger channels, variations in the heat transfer rate, airflow distribution, manufacturing tolerances, and gravity are the dominant factors. Furthermore, a recent trend of heat exchanger performance enhancement promoted miniaturization of its channels, which in turn negatively impacted refrigerant distribution. Since it is extremely difficult to control all these factors, along with the complexity and inefficiency of the proposed techniques or prohibitively high cost of the solutions, many of the previous attempts to manage refrigerant distribution, have failed.
On the other hand, refrigerant maldistribution may causes significant heat exchanger and overall system performance degradation over a wide range of operating conditions. Therefore, it would be desirable to reduce or eliminate refrigerant maldistribution in parallel flow heat exchangers.